1. Field of the Invention
The present invention relates generally to air conditioning control systems and more specifically to a system for controlling the position of air conditioning dampers as a function of two sensed parameters.
2. Description of the Prior Art
Variable volume air conditioning systems provide for heating and cooling rooms, or zones of rooms, by adjusting the rate of flow into each room or zone. The temperature of the incoming air remains constant, with only the rate at which air moves into the zone being variable.
A number of devices for positioning dampers in ducts to control air flow have been developed. When the room temperature increases or decreases relative to the desired value, the movable dampers are opened or closed to adjust the volume of air flowing into the zone. The speed of the motor driving the dampers can be made variable to improve the response time of the system.
The accuracy of conventional systems is limited due to the substantial feed back delay between operation of the dampers and a change in room temperature. This causes the dampers to be overdriven. For example, if the room becomes too hot the dampers are opened to increase air flow and lower the temperature. When the room reaches the desired temperature, the dampers are closed back to their normal position. However, the temperature of the room will continue to fall for a short period of time. This causes the dampers to close further to warm the room. Usually, the room temperature will overshoot the desired setting several times before settling down. Such excursions of room temperature often define a damped oscillation.
A characteristic of present systems is that the room temperature variable gives rise to a certain damper position. However, various static pressures and flow rates in the system of a building cause the damper position to inaccurately indicate the volume of air flowing into the zone. That is, for a given damper position, the volume of air flowing into the zone can fluctuate over a wide range.
Present systems employing a variable speed damper motor are somewhat limited in that the motor speed is proportional to the difference between the room temperature and the desired temperature. Thus, if the room temperature suddenly changes, the dampers may be quickly driven fully opened or fully closed. This tends to magnify the oscillation effect discussed above.
Present variable volume control systems are characterized by limited accuracy and slow response time to bring the room back to the preselected temperature because the input signal consists of only one variable, that being the temperature of the room. The effect of having the only system feedback parameter increases these inaccuracies.